Previous studies from our laboratory demonstrated that over one- half of asymptomatic patients with hypertrophic cardiomyopathy have silent, stress-induced perfusion defects on thallium-201 tomographic scanning. Repetitive episodes of myocardial ischemia may eventuate in myocardial damage and later left ventricular dysfunction, or provide a setting for genesis of malignant dysrhythmias. In the present investigation, we studied the efficacy of the calcium channel blocker verapamil in preventing these stress-induced perfusion abnormalities. We found that the majority of patients with perfusion defects off medication had improved regional myocardial perfusion with verapamil, and in many cases perfusion became completely normal. These data have important therapeutic implications for this group of patients, and long-term follow-up will attempt to define the prognostic aspects of the finding of silent ischemia.